A novel chromatographic system is introduced. The system internally generates a concentration gradient of ammonium sulfate (AS) along a long channel to fractionate proteins according to their solubility in AS solution. The separation column consists of a pair of disks with mutually mirror-imaged spiral channels that are separated by a semipermeable membrane. The disk assembly is mounted on a sealless continuous flow centrifuge. Concentrated As solution is introduced into the upper channel while a water solution is passed through the lower channel in the opposite direction in a rotating column. A mixture of proteins injected into the water channel moves along an AS gradient of increasing concentration that has been established in the water solution. Each protein species precipitates at a different AS concentration along the gradient. The eluate is continuously monitored and collected using a fraction collector. The method has been demonstrated on separation of serum proteins and applied to purification of a recombinant ketosteroid isomerase from a crude E. coli lysate by adding an affinity ligand to the sample solution.